Protective circuit



July 15,` 1958 L.. s@ LAPPIN PROTECTIVE cmcum Filed June 24, 1955 WNYUnited. drittes liiatent @fine i* lidif Patented .July l5, i958 PRTECT ECMUET Lester S. Lappin, Merchantville, N. il., assigner to RadioCorporation of America, a corporation of Delaware Application .lune 24,1955, Serial No. i7,733

11 Claims. (Cl. lwi) This invention relates to a protective circuit, andmore particularly to a circuit for protecting the high voltage powersupply and the modulator and power amplifier stages of a radiotransmitter from the damaging effects which might otherwise occur as aresult of overloads in the modulator or power amplifier stages, or as aresult of arc backs in the rectifier tubes of the power supply.

In a high power rectifier using mercury vapor rectier tubes andsupplying a load in a radio transmitter, the currents which flow underfault conditions may be eX- tremely high and the power dissipated underthese conditions may be sufficient to cause considerable damage toeither the rectifier or the load, unless the power is removed fastenough. Prior protective circuits ordinarily used overload relays,circuit breakers and contactors, all of which require 0.1 second orlonger after the occurrence of the fault, before the circuit is internrupted. During this time interval fault currents as high as twenty-fivetimes normal may flow.

An object of this invention is to devise a novel protective circuit fortransmitters which is extremely fast acting, both on overload and on arcbacks in the rectifiers of the power supply.

Another object is to devise a novel protective circuit for transmitterswhich acts to recycle after a fault,

so that in the event a fault is cleared within a certain time, such astwo seconds, the power is automatically restored.

A further object is to provide a protective circuit for transmitters,with means for minimizing the effect of alternating currents which flowin the transmitter tube load, on the overload protective circuit.

A still further object is to devise a novel fast-acting protectivecircuit for transmitters which is relatively simple and inexpensive.

The objects of this invention are accomplished, briefly, in thefollowing manner: A relay is connected to be operated in response to anoverload, the operation of this relay increasing the bias on therectifier tubes of the power supply so that they do not conduct, thusimmediately removing power from the load. This relay drops out as soonas the rectifier output drops to zero, to begin the recycling and toallow the bias on the rectifier tubes to be decreased to the point ofconduction at the end of a time interval determined by the dischargingof a capacitor. If the fault is cleared, normal operation is resumed,but if it persists the relay will again operate and the above-describedsequence of operations will be repeated until a lockout relay operatesat the end of a predetermined time interval to increase the bias on therectifier tubes and to maintain it increased, so that the rectifieroutput remains at zero. Another relay is connected in combination with arectifier in such a way that arc backs in the rectifier tube or tubesoperate this other relay to increase the bias on the rectifier tubes andstop conduction therein, the recycling then being initiated by thisother relay when it drops off, as soon as the rectifier output drops tozero. For arc backs,

the lockout relay circuit then operates in the same way as previouslydescribed.

The foregoing and other objects of the invention will be betterunderstood from the following description of an exemplification thereof,reference being had to the accompanying drawing, wherein the singlefigure is a circuit diagram illustrating the protective circuit of thisinvention.

Referring to the drawing, the primary winding of a plate transformer 1is connected to the alternating current (A. C.) power supply mains, andopposite ends of the secondary winding of this transformer are connectedto the respective plates of two grid-controlled gaseous rectifier tubes2 and 2', which may for example be type 5563 thyratrons. The alternatingcurrent in the A. C. mains may be 60 cycles per second, for example. Therectifier circuit is illustrated as a single-phase fullwave circuit, butit is desired to be pointed out that the protective circuit of thisinvention can be used with any conventional rectifier circuit. Themidpoint of the secondary winding of transformer ll is connected througha choke il to ground, to constitute the negative terminal of therectifier power supply. The primary windings of transformers 3 and 3supplying heating energy to the filaments of tubes 2 and 2 are bothconnected to the A. C. power supply mains, the secondary winding oftransformer 3 energizing the filament in tube 2 and the secondarywinding of transformer 3 energizing the filament in tube Z. A resistor 4is connected from the midpoint of the secondary winding of transformer 3to the rectifier output lead 42, and a resistor 4i is connected from thesecondary winding of transformer 3 to the rectifier output lead ft2.Lead 42 thus constitutes the positive output lead of the rectifier powersupply, and the output current of the rectifier fiows in parallelthrough resistors 4 and 4 to lead 42.

From lead 42, the rectifier output current flows through a resistor S toa first load represented schematically by a resistor 7. This first loadmay, for example, be the modulator tube or tubes of a radio transmitter.From lead 42, the rectifier output current also flows through a resistor6 to a second load represented schematically by a resistor 3. Thissecond load may for example be the power amplifier tube of the radiotransmitter.

Direct current bias is supplied to the control grids 9 and 9 of therespective rectifier tubes 2 and 2 by the bias supply lil, which is anysuitable unidirectional power supply having positive and negativeterminals as illustrated. A voltage divider consisting of two resistorsil and i12 is connected from the negative terminal to the positiveterminal of bias supply lh, and the common junction of these tworesistors is connected to lead i2 and thus to the filaments of tubes 2and 2. To complete the biasing circuit, a lead 43 connects the ativeterminal of bias supply l@ to the midpoint of secondary winding of apulse transformer 13 and the opposite ends of this secondary winding areconnected to the respective grids 9 and 9'. Under normal conditions,then, the bias supplied to grids 9 and 9 is that developed acrossresistor ll. This direct current bias is of sufficient magnitude toprevent conduction in tubes 2 2' in the absence of an alternatingcurrent voltage on the control grids 9 and 9.

Tubes 2 and 2 are caused to conduct at predetermined times bysuperimposing a narrow pulse on the D. C. bias. This pulse is providedby means of a pulse transformer 13 the primary winding of which isconnected to the A. C. mains, and such pulse is of a magnitudesufiicient to override the cutoff D. C. bias on tubes 2 and 2'. Thepulse results from the unsaturated condition of pulse transformer 13occurring when the A. C. mains current is going through zero; the coreof this 3 transformer is saturated at all other times during the A. C.cycle.

From output lead 42, a connection extends through the operating coil ofa relay 16 and through an adjustable resistor 14 to a tap; on resistor5, so thata portion of the current drawn by load 7 flows through thecoil of relay 16. In normaloperation, a resistor 14 is adjusted so thatrelay f6 will operate in response to a current flowing through load 7which is slightly above the normal current for this load, but will notoperate in response to the normal load current. When an overload occursin load 7, an excess current flows through the coil of relay 16, causingit to operate. Relay f6 has two pairs of normally-open contacts 1S and,23, which are closedk when this relay` operates. close,.a circuit forenergization of` an indicator relay l?? is completed as follows:Positive terminal of a relay power-supply 20 (which may be any suitablelow-voltage unidirectional source), now-closed contacts i8, the coil ofrelay 19, normally-closed manually-operated reset switch 31 and lead 44to the negative terminal of relay power supply 20. Power supply 2t) alsosupplies operating power for lockout relay 25, to be later referred toin detail, and holding power for indicating relays 46 and sie', also tobe later referred to in detail.

Relay i9 has two pairs of normally-open contacts 2l and 29, whichareclosed when this relay operates. When holding7 contacts 21 close, aholding circuit for relay 19 is completed as* follows, causing thisrelay to remain operated: Positive terminal of power supply 20,nowclosed contacts 21, coil of relay 19, normally-closed reset switch 3land lead 44 to the negative terminal of power supply 2t?. An indicatinglamp 22 is connected across the coil of indicator relay 19. This lamp islighted when relay 19 is energized or operated, due to the voltage dropacross this relay coil. Thus, indicating lamp 22 lights to indicate thepresence of an overload in load 7 (load No. l).

Relay 16 is extremely fast-acting and it operates substantiallyinstantaneously upon the occurrence of an overload in load '7. Whenrelay f6 operates, its contacts 23 also close to short-circuit resistorl2, since one contact of this contact pair 23 is connected to lead 42and the other contact of pair 23 is connected tothe positive terminal ofbias supply l0. The sho-rt-circuiting of resistor l2 causes the fullvoltage of bias supply l@ to appear across resistor lll and a capacitor24 which is connected in parallel therewith. It will be remembered thatthe bias applied to grids 9 and S is that developed across resistor 1i.This bias is increased when resistor l2 is short-circuited, so that thebias then applied to control grids 9 and 9 is increased to the fullvoltage of bias supply l0. Themagnitude of this voltage is such that thepulses generated by pulse transformer i3 are no longer suflicient tocause conduction in the tubes 2 and 2', so that the rectifier output (atlead 42) is reduced to zero. Since contacts 23 are closed substantiallyinstantaneously upon occurrence of an overload in load 7, and since thecutting-off of the rectiers 2 and 2 is then effected with no delay,solely electronically by the application of increased grid bias, it maybe seen that the power is removed from the load almost instantaneouslyupon the occurrence of an overload in load 7. In fact, the speed ofoperation of the protective circuit has been found by measurement to beconsistently less than 0.5 millisecond, on short-circuit.

Power can be supplied to any number of loads such as 7 and 3, each withindividual protection and indication. Thus, load 8 may have associatedwith it the calibration resistor l5, an overload relay 17 (not shown)similar' to relay 16 and arranged in a manner similar to relay i6, andanother indicator relay similar to relay 19, together with an indicatinglamp similar to lamp 22. rl'fhis other indicator relay is arranged in amanner similar to relay i9. In the figure, the indicator relay and Whencontacts indicating lamp associated with overload relay 17 are notshown. The particular indicating lamp such as 22k which is lit indicatesthe location of the fault, or where the overload condition occurred.

When the rectier output is reduced to zero by the cutting-off ofrectiers 2 and 2. there is no longer any energizing vo-ltage for relay16, so that this relay becomes deenergized or drops out, opening itscontacts 23 and removing the short-circuit from across resistor t2.Relay ,te is not energized from relay supply 20 ecause, forenergization, one end of the coil of this relay must be grounded, andneither terminal of power supply Ztl goes to ground; as far as relay 15is concerned, both terminals of supply 20 are essentially at the samepotential. This removal of the short-circuit from across i2 decreasesthe effective applied voltage on capacitor 24, and capacitor 24 thenstarts to discharge at a rate determined by the time constant ofcapacitor 24 and resistor l1. After a period of time determined by thistime constant, the bias voltage on grids 9 and 9 (that is, the voltageacross resistor 11 and capacitor 24) drops to a value low enough toallow conduction in tubes 2 and 2. This provides automatic recycling. Ifthe fault has then cleared (overload conditions no longer existing inthe load) normal operation is resumed. However, if the fault p-ersistsrelay 164 will again bc operated as before, and the foregoing sequenceof operations is repeated until a lockout relay 25 operates. Relay 2S isa time delay relay the contacts of which close approximately two secondsafter its coil is energized, so that this relay will be operated abouttwo seconds after the start of the overload, if thek fault stilllpersists.

One end of the coil of van anti-lockout relay 26 is connected to thepositive rectifier output lead 42 by way of lead 4liand the other end ofthis coil is connected: through a dropping resistor 45 to ground.Whenever voltage is present between lead 42 and ground, thaty is,whenever the rectifier output voltage is present, relay 264 energized.Relay 26 is not energized from relayI supply 20 because, forenergization, one end of the coil of this relay must be grounded, andneither terminal of power supply 20 goes to ground; as far as relay 26is concerned, both terminals of supply 20 are essentially. at the samepotential. Relay 26 is a slow-acting relay, that is, it is aslow-operate, slow-release relay, and4 it has a pair of normally-closedcontactsk 27A which are opened when relay 26 operates. During normaloperation of the rectier power supply, relay 26I-is continuouslyenergized and its contacts 27 remain open. One end of the coil of relay2:3 is connected to lead 44 and the negative terminal of relay supply2d, while contacts 27 are connected between the opposite end Vof suchcoil and lead 2S. Thus, the energization circuit for relay 25 from lead28 may be completed only through contacts 27, and when these contactsare open relay 25 is disabled.

ln the event of operation of any of the overload indicator relays,1 suchas relay 19 for example, relay power from the positive terminal `ofpower supply 20 is applied to lead 23 through the contacts 29, which arenormally open but which are closed when relay 19 operates. This sets upa possible circuit for energization of relay 25 through contacts 27, ifthe latter are closed. Relay 25 if; a time delay relay and its contactsclose approximately two seconds `after its coil is energized. lf thefault is cleared before lockout relay 25 can operate, the rectiters 2and 2' again conduct and normal operation is resumed, due to therecycling action previously described. If normal operation isautomatically resumed at any time within two seconds of the start of theoverload, relay 26 is again energized (even though it had previouslybecome deenergized as a result of the absence of voltage on output lead42), opening contacts 27 and disabling relay 25, thus preventingoperation of the lockout relay 25,

assent? to When normal operation is resumed, voltage is present onoutput lead 42, resulting in the energization of relay 26. Even thoughnormal operation is resumed automatically, the indicating lamp 22remains lit, signifying that a fault has occurred. In this case, manualopening of reset switch 31 deenergizes relay i9 and extinguishes lamp22.

During the recycling of the circuit, output voltage is present on lead42 only during a very small portion of the total time (for example, itis present less than about one-half millisecond, then absent for aboutone-quarter second), so that relay 26 dro-ps out or is deenergized,closing contacts 27. Relay 26 may thus drop out during the recycling,even though it may eventually be energized and pull up (and stay up) dueto the prolonged presence of output voltage on lead 42, prior to theoperation of lockout relay 25.

If the fault does not clear, relay 26 drops out (is deenergized) andremains out as a result of the prolonged absence of output voltage onlead 42. Deenergization of relay 26 closes its contacts 27 and appliesrelay power to the coil of relay through a circuit as follows: Positiveterminal of relay supply 20, now-closed contacts 29 of relay 19, lead23, closed contacts 27 of relay 26, coil of relay 25, lead 44 tonegative terminal of power supply Ztl. After a predetermined period oftime (the time delay of relay 25, approximately two seconds) relay 25operates, closing its normally-open contacts 30. This short-circuitsresistor 12 through the normally-closed rem set switch 3i, causing thevoltage across resistor 11 (applied to grids 9 and 9') to go to andremain at the full voltage of bias supply it), which is cutoff voltagefor the rectifier tubes. Then, the rectifier tubes 2 and 2 cannotconduct and the output voltage remains off lead 42, causing relay 16 toremain deenergized and deenergizing both of the loads 7 and 8.

Operation can under these conditions be resumed only by manuallyoperating the reset switch 31 to open the same. Opening of switch 31breaks all energizing cir-- cuits for relay 19 and this relay releasesor falls out, opening its contacts 29 and removing relay power from lead28. This in turn breaks the energizing circuit for relay 25 and thelatter drops out or releases, opening its contacts 30. Opening of switch31 also removes the short-circuit from resistor 12, decreasing thecontrol grid bias on tubes 2 and 2 and causing these tubes to lagainconduct, providing output voltage on lead 42. This in turn causesenergization of relay 26, opening its contacts 27 to break the circuitto the coil of relay 25, so that the latter would be deenergized in thisway even though contacts 29 of relay 19 were still closed.

The releasing, falling out, dropping off or deenergiza tion of relay 19extinguishes the indicator lamp 22.

The protective circuit of this invention also operates to protectagainst arc backs in the rectifier tubes. A relay 46 has an operatingcoil 33 and a holding coil 35, and

the operating coil 33 is connected between the midpoint of the secondarywinding of transformer 3 and one side of a rectifier 32. The other sideof rectifier 32 is connected through the coil of a relay 34 to outputlead 42. Thus, in parallel with resistor 4 is the series combination ofcoil 33, rectifier 32 and the coil of relay 34. During normal operation,the output current of the rectifier flows through resistor 4 in such adirection that rectifier' 32 is nonconducting, but during an arc backthe current through the tube 2 (assumed faulty) and resistor 4 reverses.

Similarly,` a relay 46 (not shown) has an operating coil 33 and aholding coil, and the operating co-il 33 is connected between themidpoint of the secondary wind-- ing of transformer 3' andone side of arectifier 32. The other side of rectiher 32 is connected through thecoil of the relay 34 to output lead 42. Thus, in parallel with resistor4 is the series combination of co-il 33', rectifier 32 and the coil owclay 34. During normal operation,

the output current of the rectifier flows through resistor 4 in such adirection that rectifier 32 is nonconducting, but during an :arc backthe current through the tube 2 (assumed faulty) and resistor 4 reverses.Relay 46 is similar to relay 46, is provided with contacts which areconnected in the same way as those of relay 46, and includes a holdingcoil, an indicating lamp, etc., just `like relay 46.

Assume that an arc back occurs in tube 2. With the reversal of current,rectifier 32 becomes conducting, and current flows through the coil 33of relay 46 and through the co-ii of relay 34, to operate relays 46 and34, this current resulting from the voltage drop across resistor 4.Relay 46 has two pairs 47 and 48 of normally-open contacts, which areclosed when this relay operates. Relay 46 then holds itself closed byvirtue of holding coil 35, through a circuit as follows: Positiveterminal of relay supply 20, now-closed contacts 47, coil 3S,normally-closed reset switch 31 and lead 44 to the negative terminal ofpower supply 20. An indicating lamp 36 is connected across the coil 35,this lamp being lighted due to the voltage drop across such coil whencoil 35 is energized. indicating lamp 36 lights to indicate the presenceof an arc back.

Relay 34 has one pair 49 of normally-open contacts which are connectedin parallel with contacts 23, so that when contacts 49 are closed by theenergization of relay 34 in response to an arc back, resistor 12 isshort-circuited. Iust as previously described in connection with contacts 23 operated by rel-ay 16, the rectier tubes 2 and 2 are thenbiased beyond cutoff and conduction in these tubes is stopped. Just aspreviously described in con nection with relay 16, relay 34 drops outwhen the rectifier output is reduced to zero, removing the short-circuitfrom across resistor 12 and beginning the `operation of the automaticrecycling, just as described above in connes-- tion with relay f6 andcapacitor 24, etc.

Also, when relay 46 is closed, relay power from the positive terminal ofpower supply 20 is applied to lead 2S through the contacts 48, which arenormally open but are closed when relay 46 operates. Just as previouslydescribed in connection with contacts 29 operated by relay 19, lockoutrelay 25 is then set up for operation in the same manner as describedabove, and relays 25 and 26 operate in exactly the same manner aspreviously described. In other words, from the operation of relays 46and 34 on, both the recycling and lockout circuits operate in exactlythe same manner as previously described. In this connection, it will benoted that the contacts 48 of relay 46 are connected directly inparallel with the contacts 29 of relay 19.

In a radio transmitter using high level modulation, load 7 may be themodulator tube or tubes and load 8 may be the power amplifier tube, aspreviously stated. In both of these load circuits, it is normal foraudio frequency currents to ow. In each case, these audio frequencycurrents must find a path to ground through the rectifier filtercapacitor 37, which is connected to ground from the common junction ofresistor 6 and load 8. These currents must find a path to ground becausethe speed of the overload protective circuit is such that it couldrespond to the lower audio frequency currents if such currents werepermitted to flow through the over load relays 16 and 17. It is notpractical to bypass both loads 7 and 8 to ground since the impedancesrepresented by resistors 5 and 6 are quite low and the bypass capacitorswould have to be extremely large for effective bypassing.

According to this invention, this problem is solved by first connectingthe filter capacitor 37 across the power amplifier 8 such that its A. C.component is bypassed to ground directly. Then, a capacitor 38 isconnected between the taps on resistors 5 and 6. If the reactance ofcapacitor 38 is small compared to the resistances 39 and 40 (which arethe parts of resistors 5 and 6 respectively Resistors 4 and 4 2.5 ohmseach. Resistors 5 and 6 l5 ohms each, tapped at 1l ohms.

Resistor 11 40,000 ohms.

Resistor 12 82,000 ohms.

Resistors 14 and 15 500 ohms each (adjustable). Capacitor 24 10 mfd.

Capacitor 37 16 mfd.

Capacitor 38 50 mfd.

With the values of components indicated, the recycling time, determinedby resistor 11 and capacitor 24, is approximately 1A second.

What is claimed is:

l. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to anabnormal condition in said circuit for increasing the bias supplied tosaid grid to a value sufiicient to cut ofi said rectifier, saidlast-mentioned means acting in response to the cutting off of saidrectifier to nullify said increase of bias, and time delay means fordelaying the reapplication of said operating 1oias for a predeterminedtime interval after said nullification has occurred.

V2. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, relay means responsive toan abnormal condition in said circuit for increasing the bias suppliedto said grid to a value sufficient to cut `Off said rectifier, andseparate means responsive to the continued existence of said abnormalcondition, at the end of a predetermined interval following the onsetthereof, for increasing the bias supplied to said grid to a cutoffvalue.

3. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to anabnormal condition in said circuit for increasing the bias supplied tosaid grid to a value sufiicient to cut off said rectifier, saidlast-mentioned means acting in response to the cutting off of saidrectifier to nullify said increase of bias, time delay means fordelaying the reapplication of said operating bias for a predeterminedtime interval after said nullification has occurred, and separate meansresponsive to the continued existence of said abnormal condition, at theend of a predetermined interval following the onset thereof, for cuttingoff said rectifier.

4. In `a power supply circuit, a grid-controlled gaseous rectifier forlsupplying unidirectional power to a load, means for supplying anegative operating bias to the grid of said rectifier, relay meansresponsive to an abnormal condition in said circuit for increasing thebias supplied to said grid to a value sufficient to cut oft` saidrectifier, said relay means acting in response to the cutting off ofsaid rectifier to nullify said increase of bias, time delay means fordelaying the reapplication of said operating bias for a predeterminedtime interval after said nullification has occurred, and separate meansresponsive to the continued existence of said abnormal condition, at theend of a predetermined interval following the onset thereof, forincreasing the bias supplied to said grid to a cutoff value.

5. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid yof said rectifier, means responsive to anexcessive current drawn by said load for increasing the bias supplied tosaid grid to a value sufiicient to cut ofi said rectifier, saidlast-mentioned means acting in response to the cutting off of saidrectifier to nullify said increase of bias, and time delay means fordelaying the reapplication of said operating bias for `a predeterminedtime interval after said nullification has occurred.

6. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to an arcback in said rectifier for increasing the bias supplied to said grid toa value sufficient to cut off said rectifier, said lastmentioned meansacting in response to the cutting off of said rectifier to nullify saidincrease of bias, and time delay means for delaying the reapplicationlof said operating bias for a predetermined time interval after saidnullification has occurred.

7. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to anexcessive current drawn by said load for increasing the bias supplied tosaid grid to a value sufiicient to cut off said rectier, saidlastnientioned means acting in response to the cutting off of saidrectifier to nullify said increase of bias, means responsive to an arcback in said rectifier for increasing the bias supplied to said grid toa value sufficient to cut ofi said rectifier, said last-mentioned meansacting in response to the last-mentioned cutting ofi of said rectifierto nullify said last-mentioned increase of bias, and corninon time delaymeans for delaying the reapplication of said operating bias for apredetermined time interval after either of said nullifications hasoccurred.

8. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to anexcessive current drawn by said load for increasing the bias supplied tosaid grid to a value sufficient to Cut ofi said rectifier, meansresponsive to an arc back in said rectifier for increasing the biassupplied to said grid to a value sufficient to cut ofi said rectifier,and common means responsive to the continued existence of said excessivecurrent condition or of said arc back condition, at the end of apredetermined interval following the onset of either of said conditions,for cutting ofi said rectifier.

9. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to anexcessive current drawn by said load for increasing the bias supplied tosaid grid to a value sufiicient to cut off said rectifier, saidlastmentioned means acting in response to the cutting ofi of saidrectifier to nullify said increase of bias, time delay means fordelaying the application of said operating bias for a predetermined timeinterval after sai-d nullification has occurred, and means responsive tothe continued existence of said excessive current condition, at the endof a predetermined interval following the onset thereof, for cutting offsaid rectifier.

l0. In a power supply circuit, a grid-controlled gaseous rectifier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to an arcback in said rectifier for increasing the bias supplied to said grid toa value sufficient to cut off said rectifier, said last-named meansacting in response to the cutting ofi of said rectifier to nullify saidincrease of bias, time delay means for delaying tlie reapplication ofsaid operating bias for a predetermined time interval after saidnullification has occurred, and means responsive to the continuedexistence of said arc back condition, at the end of a predeterminedinterval following the onset thereof, for cutting off said rectifier.

1l. In a power supply circuit, a grid-controlled gaseous rectier forsupplying unidirectional power to a load, means for supplying a negativeoperating bias to the grid of said rectifier, means responsive to anexcessive current drawn by said load for increasing the bias supplied tosaid grid to a value sufficient to cut off said rectifier, saidlast-mentioned means acting in response to the cutting ofi of saidrectifier to nullify said increase of bias, means responsive to an arcback in said rectifier for increasing the bias supplied to said grid toa value sufficient to cut off said rectifier, said last-mentioned meansacting in response to the last-mentioned cutting oi of said rectiiier tonullify said last-mentioned increase of bias, common time delay meansfor delaying the reapplication of said operating 5 predeterminedinterval following the onset of either of said conditions, for cuttingoff said rectifier.

References Cited in the le of this patent UNITED STATES PATENTS PrinceSept. 17, 1935 2,169,202 Winograd Aug. 8, 1939 2,179,308 Taliaferro Nov.7, 1939 2,257,449 Bany Sept. 3o, 1941

